The present invention relates to the separation of hydrocarbon gases into components of differing boiling points. The invention relates more specifically to a method and an apparatus especially suited for separating propane, methane or ethane from natural gas.
The applicant's prior U.S. Pat. No. 4,770,683, issued Sep. 13, 1988, describes a process and an apparatus for distillation of two materials of differing boiling points. A process for distillation of two materials of differing boiling points particularly propane, ethane or carbon dioxide from natural gas is described in which the conventional distillation tower is divided into a first tower at higher pressure than a conventional tower and a second tower at lower pressure. Liquid drawn from the first is expanded to the lower pressure through two or more stages with cool extracted at each stage and used to cool gas withdrawn from the top of the first tower to keep the top tray at a required temperature. Gas withdrawn from the second tower is compressed and cooled for return to the first tower as a reflux. The use of the cool from the expanded liquid and the use of the two towers provides an improved thermodynamic efficiency and avoids the use of costly turbo-expanders.
In addition, a further arrangement by the present applicant is disclosed in PCT published application WO95/10011 of Apr. 13, 1995. This discloses an improvement to the above patent in which efficiency is enhanced by the provision of a third tower and an arrangement by which additional cool is supplied to the top of the high pressure tower as a reflux.
Traditionally natural gas at less than 100 psig has been ignored for lpg recovery. Whenever such gas is processed, it is first compressed to above 300 psig before processing. However the process of the present invention, used for separation of various materials of close boiling points generally uses a distillation tower arrangement.
This invention is particularly concerned with separation of heavier components from natural gas.
Ethane recovery is similar to lpg recovery in concept except that more energy is required for refrigeration and reflux compression. This process also applies to situations where the low pressure gas is sold at higher pressures but the benefits compared to other processes are much less than that described in the first paragraph where, essentially there are no other processes that are ever considered unless the desired residue gas pressure for the sales pipeline is above 200 psig. The use of this technology for the recovery of ethylene in ethylene plants, will reduce the power requirements and capital cost of the Demethanizer portion of these plants. The above U.S. patent of the applicant was described as being very applicable to the separation of ethane and ethylene. That patent could also be used for the Demethanizer in an Ethylene Plant but it is believed that this patent will be an improvement when combined with that patent.
This invention relates to distillation processes for the separation of close boiling point materials. Such a process is used in the extraction of various materials generally using a distillation tower. Examples of such separations are:
1. Recovering ethane from natural gas PA1 2. Recovering propane from natural gas PA1 3. Recovering carbon dioxide from natural gas PA1 4. Recovering helium from natural gas PA1 5. Rejecting nitrogen from natural gas PA1 6. Recovering ethylene in ethylene plants.
This patent has optimal advantage when utilised in conjunction with a two tower or multi-tower process described in the above United States patent. It may also be used to advantage with other distillation patents for example the various arrangements described in patents held by the Ortloff Corporation.